Treatment of stainless steel and similar alloys to reduce hydrogen outgassing

ABSTRACT

Outgassing of hydrogen from high-vacuum system components made of stainless steel and similar alloys can be minimized if the metal surfaces to be exposed to the vacuum are treated according to the technique disclosed herein. First, an adherent layer comprising a mixture of oxides and nitrides is formed on the metal surfaces. This oxide and nitride mixture layer serves as a barrier to hydrogen diffusion. Then, a reduced layer having a low heat of adsorption for water is formed on the exposed surface of the oxide and nitride mixture layer. This reduced layer is made sufficiently thin to preclude outgassing of significant amounts of hydrogen into the vacuum system. In effect, a vacuum envelope is formed which outgasses only from a thin layer of reduced oxide.

United States Patent Hill et al.

TREATMENT OF STAINLESS STEEL AND SIMILAR ALLOYS TO REDUCE HYDROGEN OUTGASSING Inventors: Eugene F. Hill, Belmont; Jack L.

Walls, Saratoga, both of Calif.

Assignee: Varian Associates, Palo Alto, Calif.

Filed: Dec. 26, 1972 Appl. No.: 318,139

US. Cl. 148/315 Int. Cl. C23b 9/00, C23b 11/00 Field of Search 148/315, 6.35; 29/195;

References Cited UNITED STATES PATENTS Primary Examiner-L. Dewayne Rutledge Assistant ExaminerArthur J. Steiner Attorney, Agent, or Firm-Stanley Z. Cole; John J. Morrissey 57 ABSTRACT Outgassing of hydrogen from high-vacuum system components made of stainless steel and similar alloys can be minimized if the metal surfaces to be exposed to the vacuum are treated according to the technique disclosed herein. First, an adherent layer comprising a mixture of oxides and nitrides is formed on the metal surfaces. This oxide and nitride mixture layer serves as a barrier to hydrogen diffusion. Then, a reduced layer having a low heat of adsorption for water is formed on the exposed surface of the oxide and nitride mixture layer. This reduced layer is made sufficiently thin to preclude outgassing of significant amounts of hydrogen into the vacuum system. In effect, a vacuum envelope is formed which outgasses only from a thin layer of reduced oxide.

TREATMENT OF STAINLESS STEEL AND SIMILAR ALLOYS TO REDUCE HYDROGEN OUTGASSING BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is a further development in highvacuum technology, particularly with respect to hydrogen outgassing and pumping.

2. Description of the Prior Art In a discussion of the state of the art appearing in The Journal of Vacuum Science and Technology, Vol. 6, No. 1, January/February 1969, pp. 166-173, the author, D.G. Bills, concluded: ...present day vacuum systems have ultimate pressure limited not by any failure of the pumping mechanism, but by inadequate or improper processingof the'system materials. This limitation appears to be due to diffusion of dissolved gases, mainly hydrogen, out of only partially degassed portions of the systems. The rate of permeation of hydrogen through stainless steels at 371C is in the order of X cc(STP)mm/hr/cm /atm- It is known with respect to'stainless steels, from which a major share of the components of a vacuum system are typically made, that an oxide layer on the surface of such a stainless steel component serves to decrease the rate of hydrogen permeation through the surface by a factor of 10 as compared to an untreated surface. However, such oxide layers are generally nonadherent or only poorly adherent to stainless steel surfaces.

A substance known in brazing practice as greenoxide or chrome oxide adheres readily to stainless steels. This substance, which will hereinafter be described more definitively, is formed as an adherent coating on brazing jigs to prevent brazing alloys from sticking to the jig, surfaces. However, the so-called BRIEF DESCRIPTION OF THE DRAWING The drawing is a flow-chart illustrating the two-step process of this invention.

DETAILED DESCRIPTION OF THE INVENTION A vacuum system component made of stainless steel or of a similar alloy is shown by reference number 1 in the drawing. The component could, for example, be part of an ultrahigh vacuum pumping system designed to attain pressures lower than l0' torr, i.e., pressures lower than would be attainable where significant outgassing of hydrogen from the system components can occur. The component might instead be part of a hydrogen detection system used in conjunction with a sodium-cooled nuclear reactor. The sodium leak detection system of such a reactor could be designed to degreen oxide has not heretofore been used as a hydrogen diffusion barrier in high-vacuum technology because of the extremely high heat of adsorption of water on the green oxide, about 30,000 calories per mole. The high affinity for water vapor in the ambient atmosphere that is exhibited by oxides which are adherent to stainless steels has inhibited the use of such oxides in high-vacuum applications because the water adsorbed by such oxides establishes a high partial pressure of water vapor in the vacuum system.

SUMMARY OF THE INVENTION This invention provides a readily adherent coating which serves as a barrier to the outgassing of hydrogen from stainless steel and similar alloys. The coating can be applied by well-known techniques to components of vacuum systems known to the present art. Thus, by this invention, present technology can be used'in applications requiring reliable routine operation at pressures as low as 10*torr.

A stainless steel component to be treated according to this invention is first treated by a chemical process which fonns an adherent layer comprising anoxide, or more typically a mixture of oxides and nitrides, on the surface thereof. The oxide and nitride mixture layer provides a barrier to the outgassing of hydrogen by diffusion as atomic hydrogen through the metal lattice of the component. The oxide and nitride mixture layer is then treated by a chemical process which forms a thin reduced metal-like layer on the surface exposed to the tect the presence of hydrogen gas generated by the chemical interaction of leaking sodium with materials disposed in the immediate vicinity of the sodium circulation system. Such a hydrogen detection system would not function effectively if the components of the system were allowed to outgas hydrogen to a significant extent, because theoutgassed hydrogen could mask and render undiscemible any hydrogen generated as a result of a sodium leak.

Oxide coatings are known to be effective in reducing the rate of hydrogen diffusion out of stainless steels and similar alloys. Oxide coatings made by direct oxidation with air or oxygen, however, are generally nonadherent or only poorly adherent to stainless steels and similar alloys, and furthermore, tend to form porous rather than smooth coating surfaces. A porous surface is unsuitable as a high-vacuum envelope because the hydrogen can diffuse through it almost as rapidly as through unoxidized stainless steels. Furthermore, a porous surface has a high heat of adsorbtion for water. In addition, such oxides are often unstable at high temperatures. However, a particular substance that does adhere well to stainless steels and similar alloys and that is stable at high temperatures, is known to brazing practice as green oxide or chrome oxide. It seemed advantageous, therefore, to examine the properties with respect to hydrogen diffusion of the so-called green oxide."

Green oxide or chrome oxide adheres readily to stainless steels and similar alloys, and is used as a coating on brazing jigs to prevent brazing alloys from sticking to the jig surfaces. The green oxide coating is formed on a stainless steel componentby exposing the component to an atmosphere of dissociated ammonia saturated with water vapor so that the atmosphere has a dew point in the range from 10C to 19C, and heating the component in this atmosphere for approximately 20 minutes at a temperature in the range from 1,000C to 1,100C. An analysis of the green oxide coating thus formed on the stainless steel surface indicates that the so-called green oxide is a complex mixture of nitrides and oxides including the following compounds: Fe N, CrN, Ni N, NiO, and Cr O Compounds of manganese and silicon were also found as minor or trace constituents. The precise proportions of the nitrides and oxides vary with the relative proportions of the constituents of the stainless steel, and probably also with such factors as temperature and exposure time to the dissociated ammonia atmosphere. It was found that by coating a stainless steel surface with the complex mixture of nitrides and oxidesthat comprises the green oxide, the rate of diffusion of hydrogen from the stainless steel at 400C can be reduced to 25 percent of the rate of hydrogen diffusion for untreated stainless steel at the same temperature. Green oxide adheres readily to stainless steel, as shown by reference number 2 in the drawing, but unfortunately, has a high surface adsorbtion capacity to water. The heat of adsorbtion of green oxide for polar molecules such as water is especially high, being on the order of 30,000 calories per mole. This characteristic renders green oxide particularly unsuitable as the exposed surface of a vacuum envelope. Water vapor is ever-present in the ambient atmosphere, and water molecules adsorbed onto the green oxidecoating would establish such a high partial pressure of water vapor in the vacuum systemv that an unacceptably high base-line pressure for the system would result.

1 The low hydrogen permeation rate of green oxide, estimated-to be less than 5 lOcc(STP)mm/hr/cm- /atm' is a desirable property for high-vacuum sysan atmosphere of anhydrous hydrogen. Such treatment will chemically reduce the exposed surface of the green oxide coating to a metal-like layer which has a relatively low heat of adsorbtion with respect to water molecules, i.e., approximately 20,000 calories per .mole. It has been found empirically that exposure to the anhydrous hydrogen atmosphere at a temperature in the range from l,000C to l,l00C for 10 minutes will produce on the green oxide coating an outer metal-like layer having sufficient strength to withstand the mechanical stresses that occur when the component is inserted into the vacuum system, yet having sufficient thinness to render negligible the quantity of hydrogen dissolved therein. The thicknessofthe outer metal-like layer produced by a 10 minute exposure to anhydrous hydrogen at l,000C as indicated will be 0.001 millimeters or less. This thickness can be varied by varying the exposure time. The result is a sandwich structure in which a hydrogen diffusion barrier in the form of a layer 2-composed of a mixture of oxides and other compounds is sandwiched between the underlying vacuum system component 1 on the one side and a thin reduced layer shown by reference number 3 on the other side. Analysis shows that the thin reduced layer exposed to the vacuum has substantially the same metal composition as the underlying vacuum system component.

In a typical vacuum system having untreated stainless steel components, a base-line hydrogen pressure of '8 l0 torr-is obtainable at room temperature using exgen pressure at room temperature was only 2 10 torr;

and that upon heating the vacuum chamber to 400C, the hydrogen pressure rose to only 4X10 torr. This indicates a greatly reduced rate of hydrogen diffusion from the stainless steel as the result of the barrier layer being sandwiched between the thin reduced layer and the underlying stainless steel component.

The oxidation and reduction conditions described herein for sandwiching the hydrogen barrier layer between the underlying vacuum system component and the thin vacuum envelope surface may vary according to the type of stainless steel or other alloy used, the structural strength required of the component to be treated and design requirements of a given vacuum system. Such variations canbe made without departing from the scope of the present invention. It is anticipated that the technique of this invention for reducing the outgassing of hydrogen can be practiced with any metal that forms protective oxides or nitrides, particularly high chromium alloys, so that the scope of this invention is limited only by the following claims.

What is claimed is:

1. An article having a surface thereof intended for exposure to a vacuum in a vacuum system, said article comprising'a stainless steel substrate, a first layer coating a portion of said substrate, said first layer comprising an oxidized form of said stainless steel substrate and having a rate of permeation for hydrogen which is substantially less than the rate of permeation of hydrogen through said substrate, and a second layer coating said first layer, said second layer comprising a reduced form of said first layer and having a lower heat of adsorption for water than said first layer, the surface of said second layer being the surface intended for exposure to said vacuum. a

2. The article of claim 1 wherein said second layer comprises substantially the same material as said substrate and is substantially thinner than said substrate.

3. The article of claim 1 wherein said surface of said second layer comprises at least a portion of a vacuum envelope of said vacuum system. 

2. The article of claim 1 wherein said second layer comprises substantially the same material as said substrate and is substantially thinner than said substrate.
 3. The article of claim 1 wherein said surface of said second layer comprises at least a portion of a vacuum envelope of said vacuum system. 